Paints based on aqueous polymer dispersions are widely used since they have two decisive advantages compared to paints based on film formers dissolved in organic solvents: they are not combustible; and they can be handled without causing any health hazard.
The aqueous polymer dispersions may be prepared by emulsifying the desired polymerizable monomer (e.g. acrylates such as ethyl acrylate; methacrylates such as methyl methacrylate; styrene; vinyl chloride; acrylonitrile; etc.) in water. Polymerization is initiated by addition of a water-soluble polymerization initiator, typically a per-compound.
An emulsifier is used in order to emulsify the water-insoluble monomer in the aqueous reaction mixture. However the emulsifier commonly has a harmful effect in that it may render the finished coating water-sensitive.
The prior art has heretofore attempted to obviate this disadvantage. Thus the amount of emulsifier has been reduced as far as possible; but only a relatively slight improvement in water-resistance has thereby been achieved. Other attempts to obtain dispersions from which water-proof coatings can be made have included the use of certain additives or special operating procedures.
For example W. German DT-OS No. 2,365,619 discloses that water resistance can be improved by addition of 1-10 w % (based on monomer) of a hydrocarbon or hydrocarbon mixture during or after emulsion polymerization.
W. German DT-OS No. 2,410,822 describes a process according to which emulsifier-free dispersions can be obtained containing only a small amount of polyvinyl alcohol as a protective colloid. The monomers are polymerized in a mixture of water, a water-miscible organic solvent, and a water-soluble, high molecular weight compound (containing hydroxyl, carboxyl, carboxylic amide, and/or ether groups), followed by precipitation of the polymers. The disadvantage of this process is that the organic solvent then must be at least partially removed by distillation under conditions such that the resin remains in the water as a disperse phase i.e., the dispersion must not be destroyed.
W. German DT-OS No. 1,795,757 discloses a cold crosslinkable acrylic resin dispersion prepared from (i) acrylates, and optionally other monomers, and (ii) halogenated acrylic esters or halogenated methacrylic esters. Because of the presence of the reactive halogen atoms, the copolymers can be vulcanized with various systems. These resins are however unsuitable as paints; and this specification does not discuss the problem of producing, from aqueous acrylic dispersions, a waterproof glossy coating.
The idea that an improvement in the gloss can be achieved by adding water-soluble resins to paint dispersions has been disclosed by K. Haman: Development Tendencies in the Lacquer Field, Farbe und Lack 907, Vol. 81 (1975). It is found that addition of a water-dilutable acid polyester to an aqueous acrylic resin dispersion permits attainment of coatings of improved gloss compared with coating obtained by use of the same acrylate dispersion without addition of polyester. However the water-resistance was unsatisfactory after subsequent cross-linking. When the acid polyester resin was replaced by a neutral, water-dilutable hydroxy polyether resin, no improvement in water-resistance of the matte coating was noted.
It is an object of this invention to provide a glossy water-proof paint based on an aqueous acrylic resin dispersion and a process for preparing the same. Other objects will be apparent to those skilled in the art.